Reproducible IgM anti-SIII plaque-forming cell (PFC) responses have been obtained from sub-lethally irradiated recipients of normal mouse spleen cells. Low-dose tolerance can be induced in normal cells after adoptive transfer, but pretreatment of donors with low doses of SIII 72 hours before transfer does not lead to reduced SIII responses by these cells nor do such cells suppress the response of normal cells. Studies are underway to further explore conditions in which it will be possible to analyze the nature of the cell(s) and/or factor(s) responsible for the regulation of this immune response in the intact animal. Additional studies are planned to determine whether the immunosuppression previously observed with preparations of minute virus of mice (MVM) in MLR and CML reactions is directed against all cells regardless of the functional activity expressed or rather is directed against specific subpopulations of T or B cells. Assays in which this viral immunosuppression is being studied include T and B mitogen responses, antigen specific proliferative responses (to soluble antigens, H-2 antigens, and M-locus antigens) and primary in vitro PFC responses to both T-dependent and T-independent antigens.